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To establish a default value for a dependency property that is of a type derived from DependencyObject, follow these steps:

1. Define the dependency property using the DependencyProperty.Register method in the class that will use it.
2. In the same class, create a static constructor and set the default value of the property using the PropertyMetadata class.
3. Use the DependencyObject.SetValue method to set the value of the property to the default value.

Here is an example of how to do this in C#:

public class MyClass : DependencyObject
{
    public static readonly DependencyProperty MyPropertyProperty = DependencyProperty.Register(
        "MyProperty", typeof(MyType), typeof(MyClass), new PropertyMetadata(new MyType()));

    public MyType MyProperty
    {
        get { return (MyType)GetValue(MyPropertyProperty); }
        set { SetValue(MyPropertyProperty, value); }
    }

    static MyClass()
    {
        MyPropertyProperty.OverrideMetadata(typeof(MyClass), new PropertyMetadata(new MyType()));
    }
}

In this example, the MyProperty dependency property is defined using the DependencyProperty.Register method. In the static constructor of the class, the PropertyMetadata is used to set the default value for the property. Finally, the DependencyObject.SetValue method is used to set the value of the property to the default value.